Hydrofoil sailing craft

ABSTRACT

A hydrofoil sailing craft having a hydrofoil assembly comprising an outrig pivotally mounted on the hull of the craft and a pair of swept-back hydrofoils mounted on each end of the outrig, an elastic restraint attached to the outrig and to a control shaft mounted on the hull of the craft whereby the foils are permitted to pivot through an angle which is controlled by the combined effects of the elastic restraint and the rearward shift of the lift center on the hydrofoils as they rise from the water. Means are also provided for adjusting the tension on the elastic restraint and for pivoting the hydrofoils inwardly to adjust the angle of the hydrofoils with respect to the outrig and ultimately to fold the hydrofoils against the outrig for transport.

United States Patent 1191 FOREIGN PATENTS OR APPLICATIONS 7/1964 Belgium 114/665 H Cleary Feb. 5, 1974 I-IYDROFOIL SAILING CRAFI Prima Examiner-Duane A. Re er 76 1 t M. c1 PoB 541, g l 1 men or g z z s g b 41 Ox Assistant Examiner-D. C. Butler Attorney, Agent, or Firm-Charles F. Steininger [22] Filed: Mar. 23, 1972 [21] Appl. No.: 237,414 [57] ABSTRACT A hydrofoil sailing craft having a hydrofoil assembly 52 us. c1. 114/665 11, 114/39 comprising an outrig pivotally mounted on the hull of 51 Int. Cl B63b l/18 the craft and a P of p a hydrofoils mounted 53 Field f Search u 1 4 5 H, 5 R, 39 1 on each end of the outrig, an elastic restraint attached 114/66 5' to the outrig and to a controlshaft mounted on the hull of the craft whereby the foils are permitted to 56] R f ren Cit pivot through an angle which is controlled by the com- 7 P bined effects Of the elastic restraint and the rearward 3-520 267 7 1970 Cl k S H4 66 5 H shift of the lift center onthe hydrofoils as they rise 16 1011962 g 14/39 from the water. Means are also provided for adjusting 3179078 4/1965 Popkin /66 5 H the tension on the elastic restraint and for pivoting the 3:357:390 12/1967 Wray ..::::I:.... :1: 114/665 H hydmfoils inwardly to adjust the angle of the hydro- 3,150,626 9 1964 Irgens 114 665 r1 foils with respect to the outrig and ultimately to fold the hydrofoils against the outrig for transport.

25 Claims, 21 Drawing Figures PAIENIE FEB SIHM new 1 HYDROFOIL SAILING CRAFT BACKGROUND OF THE INVENTION The present invention relates to an improved hydrofoil Sailing craft and, more particularly, an improved mounting and control system for the hydrofoil elements to provide a smooth, low drag transition between displacement and foil born operation, and controlled high speed operation under rough sea conditions.

In the past, a number of attempts have been made to construct hydrofoil sailing craft for the primary purpose of increasing the speed of the craft over that of conventional sailing craft. However, such hydrofoil sailing craft have, in the past, had a number of disadvantages. Chief among these disadvantages has been the instability of the craft, particularly, due to variations in the pitch of the craft and foil angle of attack caused by changes in speed, sea conditions and pitching moment of the wind acting onthe sail. Some of the earlier craft have also been overly complex, making use of servomechanisms and linkages connected to the main hydrofoils or flaps on the main hydrofoils for control. Other earlier craft make use of changes in the angle of attack of the rudder foil to change .the pitch of the, entire craft thus controlling the angle of attack of the main lifting hydrofoils. Such systems have inherently slow control responses because of the large moment of inertia about the pitch axis and are not suitable for rough sea conditions. Another disadvantage of craft effecting control by changing the depth of immersion of the rudder foil assembly is that this assembly must then be larger and more deeply immersed than is otherwise required and this larger immersed structure produces extra drag. Still another disadvantage of prior art hydrofoil craft has been the inability of such craft to absorb dynamic loads due to rough sea conditions or impactwith solid objects. Finally, the prior art craft have been cumbersome to transport due to the fixed position of the hydrofoils and there has been no adequate means of changing the dihedral angle of the surface penetrating hydrofoils while the craft is underway in order to optimize the function of the hydrofoils under various sailing conditions.

It is therefore an. object of this invention to provide a simple angle of attack control system which responds automatically and rapidly to variations in foil immersion due to changes in speed, sea conditions and changing pitching moment ofithe wind acting on the sail. A further object of this invention is to provide a hydrofoil mounting system which permits launching and recovery from a beach without damage to the foil elements. A still further object of this invention is to provide a hydrofoil at the stern which will hold the stern at nearly constant elevation after medium and high speeds are attained, even under rough sea conditions and at the same time maintain low drag on the immersed elements at the stern. Constant stern elevation permits finer control over the angle of attack of the forward hydrofoil elements and mimimum immersion of the rudder foil dihedral angle of the main surface penetrating hydrofoils while under way in order to optimize their function under various sailing conditions.

SUMMARY OF THE INVENTION The present invention relates to a hydrofoil sailing craft having an outrig pivotally mounted on the hull of the craft, a pair of hydrofoils mounted on the ends of the outrig and an elastic-restraint operatively attached to the outrig and to the hull of the craft whereby the foils are permitted to pivot through an angle controlled by the combined effects of the restraining means and the hydrodynamic forces on the hydrofoils which change accordingto conditions, in position, magnitude and direction.

BRIEF DESCRIPTION OF THE DRAWINGS The objectives and advantages of the present invention will be clear from the following description when read in conjunction with the drawings wherein:

FIG. 1 is a profile view of the hydrofoil sailing craft;

FIG. 2 is a top view of the forward portion of the hull and starbord main hydrofoil;

FIG. 3 is a rear view showing the starboard half of the hydrofoil assembly;

FIG. 4 is a rear view showing the starboard hydrofoil in collapsed position;

FIG. 5 is a view of the outrig pivot bearing connection to the hull;

FIG. 6 is a rear view showing the outrig pivot bearing connection to the hull;

FIG. 7 is a profile view of the outrig pivot beam assembly in center position;

FIG. 8 is a profile view of the outrig pivot assembly displaced through angle a FIG. 9 is a top view of the pivot beam assembly;

FIG. 10 is a rear view of the pivot beam assembly;

FIG. 11 is a profile of the stern, rudder and rudder foil assembly;

FIG. 12 is a view from behind of the stern rudder and rudder foil;.

FIG. 13 is a side sectional view of the upperrudder pivot assembly;

FIG. 14 is a top view of the rudder pivot;

FIG. 15 is a sidesectional view of the lower rudder v pivot;

structure at high speed with attendant low drag. Fine control is essential to efficient operation at high speed. Yet another object of this invention is to provide a hydrofoil mounting which is highly resilient and thus able to absorb dynamic loads due to rough sea conditions or impact with solid objects. Another and further object of this invention is to provide ameans for changing the FIG. l6is a top sectional view of the lower rudder pivot;

FIG. 17 gives four views of the main hydrofoil as a two dimensional schematic illustrating variables significant to control characteristics; including:

a. plan view I b. end view D c. edgewise view-from behind (I. profile view from starboard;

FIG. 18 shows the relationship between the im-' hydrofoil craft; and

FIG. 21 shows variations of the control angle with rearward shift of lift center position.

DETAILED DESCRIPTION A profile view of the hydrofoil sailing craft is shown in FIG. 1. The hull has a mast 12 mounted near the bow which carries a sail 14. Many styles of sail and rigging might be employed. However, as with other high speed sailing craft, a relatively stiff flat sail is preferred. The fore stay 16 guys the mast 12 to the bow and may also carry sail. The after stay 20 guys the mast 12 to the stern, and shrouds 18 guy the mast 12 to the ends of the outrig 22. The outrig 22 is pivotally mounted on the hull. The pivot axis 23 of outrig 22 is transverse to the hull and lies below the low speed water line 42, and forward of the center of gravity 27 of the hydrofoil craft. The longitudinal distance from the center of gravity 27 to the pivot axis 23 should be between 6 percent and 24 percent of the distance from the center ofgravity to the stern and preferably about l2 percent of this distance. Thus, while the craft is supported mainly by the forward hydrofoils, enough weight is supported at the stern to prevent the craft from pitching on its nose in a gust of wind.

FIG. 2 is a top view of the starboard outrig hydrofoil assembly and the forward section of the hull. FIG. 3 is a rear view of the hydrofoil outrig assembly. The foils '28 are hinged to the ends of the outrig 22 by means of hinge 54 whose axis runs substantially parallel to the long axis of the hull. Various configurations of foil may be used for the main hydrofoil elements of the invention. However, the uniformly tapered foil is especially suitable, and the discussion of the control system will be simplified by assuming uniform taper. The spans of foils 28 are disposed at a dihedral angle (b to the horizontal so that the free end projects downwardly and inwardly. The foils are braced downward at dihedral angle (b by struts 56. The dihedral angle, when the foil are in operation, will be between 30 and 50 and preferably about 38. However, the optimum angle will depend on sailing conditions. The struts 56 are joined to the foils 28 near mid span by a hinge type connection 57 having minimal frontal area and a low drag cross section. The struts 56 also have a low drag cross section and are joined at their upper ends by a hinge type connection 55 to the strut bushings 58. The strut bushings 58 have a free sliding fit on the tubular outrig means 22.

The strut bushing 58 is'held in the desired position by the strut positioning cord 59 which is stretched out along the outrig in a loop. The outer end of the loop passes over a sheave 60 near the end of the outrig. The inner end of the loop passes over a cleat 61. The ends of the positioning cord are attached to bushing 58. Bushing 58 is moved to a position giving the desired dihedral angle by pulling on the appropriate side of the loop. It is set in position by fixing the cord on the cleat 61. By pulling bushing 58 inward, the hydrofoil 28 may be folded up against the outrig for ease in launching and recovery, as shown in FIG. 4.

The overall dimensions of a practical hydrofoil craft are a compromise between several conflicting factors. If limitations on strength of materials, economies of structure and maximum draft were not taken into consideration, then for maximum stability and speed, the hull would be longer, the outrig wider and the foils longer and more slender for a given size sail than a practical craft, such as that illustrated in this disclosure. A preferred embodiment of the hydrofoil craft which hasbeen builtand tested has a hull 18 ft. long and outrig width of 16 ft. The main hydrofoils are 6 ft. long.

Each has a lifting area of 5% ft. The weight of the craft, not including crew, is 170 lbs. The ft. sail hasassnwr 9 .lzrs sl le itab the ris- T i r portion between the height of the center of wind pressure and the outrig breadth results in a craft whose lateral stability must be augmented by shifting crew weight to windward. A hiking deck or seat 72 projecting laterally from the forward part of the hull is provided for this purpose. The overturning moment due to wind pressure on the sail could be completely compensated by hydrodynamic forces on the foils if a wider outrig were used. But it is an agreeable compromise to compensate for a portion of the overturning moment by shifting the crew weight to windward and having an outrig of more reasonable width.

The main problem in operation of a hydrofoil sailing craft is the control of pitch to prevent rapid climbing and diving, and to maintain an efficient angle of attack. Variations in the pitching moment on the sail and wave action aggravate these problems. The present invention provides automatic control of the angle of attack of the foils 28 in response to changing depth of immersion and unsteady loading due to wind and wave action. If the hydrofoils 28 were rigid with'respect to the hull, elevation of the forward end of the hull would result in a large increase in the angle of attack. This can easily be visualized by inspection of FIG. 1. If, for example, the hull was 18 ft. long andthe bow rises 2 ft. relative to the stern, a rigidly connected hydrofoil system will cause an increae in the angle of attack by 3.2". However, what is actually required for efficient high speed operation of hydrofoils 28 is a reduction in angle of attack as speed increases. Low angles of attack are required for surface penetrating foils at high speed in order to prevent ventilation. As is well known, ventilation is the flooding of air over the top low pressure surfaces of the foils with attendant sudden loss of lift. Because of the ventilation problem, angles of attack must be kept small for efficient operation at high speed.

In accordance with the present invention, the foils 28 are permitted to pivot through an angle which is controlled by the combined effects of elastic restraining means 24 and the rearward shift of the lift center on the main hydrofoils 28 as they rise from the water. This rearward shift of the lift center as the foils 28 lift out of the water, results from their swept back orientation. This rotation of the foils 28 reduces the angle of attack a, as the speed increases. An efficient low speed angle of attack is about 50. At high speed, with the hull fully above the water and with a small portion of the main foils 28 penetrating the surface of the water, the attack angle should be close -to 1.

FIG. 1 shows how the boat pitches backward at an angle a, as the boat moves from low speed to high speed. The control system must compensate for pitch angle 01,. For this to be possible, a, should change in a fairly regular manner, especially at high speed when the attack angle must be controlled within narrow limits. The rudder foil assembly, FIGS. 11 and 12, elevates the stern only a small amount and then at medium and high speed holds the stern at nearly constant elevation, because the lifting foil 32 on the rudder 30 is only a short distance below the stern planing surface. The rudder foil 32 should be of a cross section suitable for planing on the surface as well as running fully submerged. A

face, and a thickness to cord ratio less than one-tenth.

At high speed, the rudder foil runs at nearly constant elevation alternately breaking surface and submerging with passing waves.

A model of the present invention has been tested with and without a lifting foil on the rudder. Without the rudder foil, the-stern remains planing on the surface. This is a workable system which falls within the present invention but is not the preferred embodiment because of lower speed potential, especially in rough water.

Referring now to FIGS. 11 and 12 which show a preferred embodiment of the rudder assembly, a rudder 30 is pivotally mounted at the stern on a substantially vertical axis. The rudder carries rigidly mounted hydrofoil 32 substantially above its lower end. Steering is accomplished by rotation of steering bar (FIG. 1). Steering cables 34, made of nylon or the like, run along the sides of the hull from the steeringbar 36 to the cross beam 38, which projects from the sides of the rudder. Rudder 30 is mounted at the top on a double pivot connection 67 detailed in FIGS. 13 and 14. The rudder engages a vee plate 73 mounted on the trailing edge of the stem planing surface as shown in top sectional view, FIG. 16.

Tension in the steering cable 34 holds the rudder in proper engagement with vee plate 73 and support bushing 78, but has sufficient elasticity to allow the rudder to swing backward and upward if it strikes a submerged object. The double pivot assembly shown in FIGS. 13

- and 14 includes a'vertical pin 75 which is threaded into the top of the rudder and accommodates its steering motions. The vertical pin is hinged to a longitudinal pin 77 by means of hinge pin 76. Longitudinal pin 77 slidably engages support bushing 78 and'bottoms against The need for placement of the pivot axis well below water line 42 makes it awkward to use a conventional type pivot bearing. For structural reasons, the pivot supports should be spread wide on the outrig, and, since a narrow rounded vee-type hull will generally be used, placement of the pivot supports deep inside the hull is precluded. Thus, it is a problem to place the pivot axis 23 at the preferred elevation and at the same time avoid drag producing structure below 42. One solution of this problem is detailed in FIGS. 5 and 6. This is a bearing set consisting of a journal segment 80 bolted to pivot arm 49 and a plate 81 with a concaved bearing surface bolted to the hull. During operation of the hydrofoil craft, the bearing surfaces are held in engagement by shoroud tension and uplift on the hydrofoils. As seen in FIGS. 5 and 6, 23 falls well below the bearing structure. This type of pivot bearing has the limitation that the large radius bearing, which may be required to position 23, will introduce excessive friction unless a more complicated roller type bearing is used. A suspension system not subject to this limitation will now be detailed.

' The system will be termed the pivot beam assembly. In this system, the outrig is pin-connected to the ends of two flat beams (aluminum or steel) which run transversely through reinforced openings in the upper part of the hull. The pivotal motion is accomplished, not by bearing rotation, but by the flexure of the two beams. FIGS. 7, 8, 9 and 10 show. the pivot assembly inside, top and rear sectional views, respectively. The pivot beams 62' have a rectangular section with a thickness,

. t, less than one-fifth'the depth, d. The cantilevered poradjusting screw 74. Adjusting screw 74 is used to change the angle of attack of the rudder foil by changing the longitudinal position of the top of the rudder. The support bushing incorporates an eyeplate 79 for attaching the afterstay and flanges 80 for attachment to the top of the hull at the stern. FIGS. 15 and 16 are top and side sectional views of the rudder engagement with the vee plate 73. The vee plate 73 is screwed to the bottom of the transom 69.

The hydrofoil assembly consists of foils 28, struts 56, outrig 22 anda pivot means. The pivot means may consist of a pivot arm and bearing set such as indicated in FIGS. 3', 4, 5 and 6 or a pivot assembly as shown in FIGS. 7, 8, 9 and 10. The pivot assembly ofFIGS. 7, 8, 9 and 10 is the preferred embodiment as will be explained. The hydrofoil assembly is able to rotate through a limited angle, a which we shall call the control angle about the pivot axis 23. The pivot axis 23 lies parallel to and below the outrig means 22. Rotation of the outrig assembly is resisted by an adjustable elastic restraining means 24, such as'nylon cord or the like. The elastic restraint pulls backward on the top of the outrig means 22 tending to hold it against stop 29 thus imparting a clockwise moment about the pivot axis to the hydrofoil assembly as viewed in FIG. 1. A preferred embodiment of the elastic restraining means is shown in FIGS. 1 and 2. By turning a knob 37, the cord 24 can be wound or unwound from a shaft 39 mounted on the steering bar 36. The cord runs through a sheave 26 on top of the outrig back to a fastener on the deck.

tions of the beams project from the sides of the hull and taper to pin ends 64 which engages the ends of brackets 63. The brackets 63 are rigidly joined to 22 so that 22 is supported above the hull with a reasonable amount of clearance. The stiffness ratio of a rectangular beam in resisting edgewise loading'as compared with flatwise loading is equal to the square of the depth over thickness ratio, ti /t Thus, a rectangular beam whose depth is eight times its thickness will be 64 times as stiff in edgewise loading. The pivot beams are mounted so that the mid planes intersect at the desired pivot axis 23. The included angle, P, between the flat sides of the pivot beams should be between 20 and 60 and preferably about 45. With the beams edgewise to the pivot axis, the beams ends are stiff and strong in the radial direction, but flexible to movement which approximately oirou a arow bout 234- h sc o motion about 23 is shared by the outrig assembly which is connected by means of brackets 63 to the ends of the pivot beams. Since the pivot beam assembly inherently places elastic restraint -on rotation of the hydrofoil assembly about its pivot axis, the adjustable elastic restraining means 24 becomes auxillary to this primary elastic restraint and must be sized accordingly. In addition to allowing arbitrary vertical positioning of 23, the pivot beam assembly has important structural advantages. Thewide spacing of the connecting brackets reduces the bending moments in the outrig means 22, permitting lighter construction. The pivot beam assembly is highly resilient and thus, able to absorb a variety of dynamic loads on the foil system.

DESIGN PARAMETERS following discussion of the relationship of the design parameters with relation to FIGS. 17, l8, l9 and 20, where:

a hydrofoil angle of attack degrees a, hydrofoil assembly angle of attack degrees at, rudder foil angle of attack degrees oz hydrofoil assembly angle of attack at zero speed degrees a rudder foil angle of attack at zero speed degrees d) dihedral angle of main surface penetrating hydrofoils degrees sweep back angle degrees A immersed area of main hydrofoils ft.

C cord length ft.

0' pivot beam depth D length from rudder foil to pivot axis ft.

C lift coefficient dimensionless C moment coefficient dimensionless g acceleration of gravity 32.2 ft/sec K, proportionality constant dimensionless K torsional spring rate degrees/ft.lb.

L lift on hydrofoils lbs.

L, vertical lift on hydrofoil assembly lbs.

M moment foot pounds t pivot beam thickness ft.

V velocity ft./sec

Y longitudinal distance from lift center to pivot axis ft.

Y longitudinal distance from low speed lift center to pivot axis ft.

AY longitudinal shift or lift center ft.

Z vertical distance from pivot axis to low speed water lineft.

h, vertical lift of pivot axis from low speed position h vertical lift of rudder foil from low speed position ft.

h vertical distance from low speed water line to lift center ft.

R, vertical distance from low speed water line to low speed lift center ft.

FIG. 17, (a), (b), (c) and (d), is a two dimensional schematic of the main starboard hydrofoil in four views. The figure shows the relationships between variables entering into our discussion of the control system. View (a) shows the plan form of the hydrofoil. The bold arrow shows the direction of motion. The taper angle of the foil is T. The sweep back angle 0, is the angle between the one-fourth cord line and a plane perpendicular to the direction of motion. The sweepback angle is important to the functioning of the control system. The greater the sweep angle, the greater the control torque available for, a' given change in immersion depth. However, if the sweep angle is too large, hydrodynamic efficiency will suffer and the torsional stresses on the hydrofoil assembly will become excessive. Accordingly, the sweep angle should be between 8 and 25 preferably about 14. FIG. 17 (b) is an end view of the foil on a plane perpendicular to the direction of motion. The true angle of attack, a, is projected in this view, FIG. 17 (c) is a view from the rear showing the dihedral angle 4). View (d) is a profile of the hydrofoil as viewed along axis 23. The angle of attack, 01,, seen in this view is a component of the true angle of attack, a, projected on a vertical plane. a, is the component that determines vertical lift and will be termed the angle of attack of the hydrofoil assembly. The relationship between the dihedral angle, (15, the true angle of attack, a, and the hydrofoil assembly attack angle, 11,, is as follows:

qhitilliq afl Lift on a hydrofoil can be calculated from the following relation:

L CL A g) In the English system of units, A is the foil area in ft, V is the velocity in ft./sec., g is the gravitational constant 32.2 ft/sec. and W is the specific weight of water. Taking W for sea water as 64.4 lb/ft then W/2g 1 and so for the special case of sea water, equation (2) can be written:

L C AV The lift coefficinet, C in equation (2) and (3) is dimensionless and is directly proportional to the angle of attack for the usable range of angles. For the hydrofoil plan form shown in FIG. 17, the relationship between attack angle and lift coefficient is approximately:

where a is measured in degrees.

Then from (3) and (4):

L 0.08 01 AV To obtain the vertical lift on the main hydrofoil assembly, we must take the dihedral angle, d1, into account. The vertical lift L, is given by:

where C is the cord length, and where C,,, is the moment coefficient. C is a constant determined by the shape of the mean line. A typical value of C for hydrofoils,

used in this invention is 0.03. The approximate locus The longitudinal shift of thelift center with elevation of the hydrofoil is given by:

1 2 I ig-(1, 191% The immersed area, A of the two main surface penetrating hydrofoils changes with the elevation of the pivot axis relative to the rest position. The relationship for the immersed area is given by:

Mit

Y is the longitudinal distance from the pivot axis to the lift center, A Y is the total change in the longitudinal position of the lift center from its low speed position, and Y,,is the longitudinal distance from 23, the pivot axis of the hydrofoil assembly, to the low speed lift center. i

The moment, M, exerted by the lift on the hydrofoil assembly about 23 is then:

The angular deflection of the assembly about 23 is proportional to this moment: I

v K2 L1 1 l o) where a, is the control angle, and K is the torsional spring rate of the hydrofoil assembly about 23. In the case of the preferred pivot beam system, the elastic restraint is largely provided by the pivot beams themselves. I

The change in the pitch angle of the hull, a,,,

is determined by the relative elevation of the pivot axis, h,, and the stern, hd thus:

a, Arc tan [h, h j/D where D is the longitudinal distance between 23 and the rudder foil.

Moving from low speed 'to high speed, the control system progressively changes the angle of attack from high values at low speed'to the required low values at high speed. The angle of attack, 01,, of the hydrofoil assembly is given by: I

9 111i CHIP where a,, is the pitch angle a is the control angle and a is the initial attack angle. The angle of attack of the rudder foil, :1 increases progressively with pitch:

(X2 612 aP M (l8) The foregoing relationships have been used to calculate the sequence of events from take off to high speed for an assumed initial hydrofoil craft configuration similar to the craft which has been built and tested. The results of these calculations are plotted in FIG. 20. In the lower speed range, the calculations are complicated by the need for data on the hull displacement for different values of h, and k During this intermediate speed range, whena portion of the hull is still in the water L, and L, are functions of h, and h. At high speed, the hull is clear of the water L, and L are constant and the calculation is simplified.

In FIG. 20, the elevation of 23, h,, the elevation of the stern, h the angle of attack of the main hydrofoils, 01,, and the angle of attack of the rudder foil, a are plotted against speed in ft./sec. h, increases rather continuously with speed. The elevation of the stern increases over an intermediate speed. range, then holds nearly constant with the foil running near the surface. The angle of attack, 01,, increases at first from 4 to 5, then decreases steadily with speed. The angle of attack of the rudder foil, 01,, changes in proportion to the pitch angle which takes a dip as the stern becomes elevated.

The position of 23 has a large effect on the response of the control system to unsteady conditions, such as, waves and gusting wind. If, for example, 23 is too high above the support center, a sudden drag increase due to running into a wave will produce a torque that cannot be resisted by available control torque. The result will then be an.uncontrolled dive. This torque, due to sudden drag increase, is proportional to the elevation of the pivot axis. Accordingly, the pivot axis 23 should be well below the low speed water line 42 and preferably:

' tioned earlier,L; ft. 1 1 ft., and letting 38 then'f'roin (911 1587i. The biErTQTlirirTsBfiZ would then 'be 0.42 ft Z 2 ft.

The longitudinal position of 23 strongly affects the behavior of the control system in response to unsteady vertical loads. The longitudinal position of the pivot axis is Y,, measured from the low speed lift center A as shown in FIG. 19. Y,, is positive when 23 is aft of the lift center and negative if it is forward of the lift center. If 23 is too far forward, a sudden increase in the vertical load on the main hydrofoils will cause the craft to dive. Such momentary increase in load may result from gusting wind or running into the face of an approaching wave. It is evident from inspection of FIG. 19 that the further forward 23 is, the greater will be the torque due to a load increase tending to diminish the angle of attack. This problem is prevented if 23 is sufficient distance aft of the low speed lift center. However, if 23 is too far aft, an opposite problem arises from a sudden vertical load increase. With 23 too far aft, the control torque, due to increased immersion depth, will overcorrect. The result is an excessive increase in the angle of attack which causes ventilation and an abrupt increase in drag. The craft will then slow down abruptly and the hull drop to the' water. The preferred longitudinal position of the pivot axis is substantially behind the low speed lift center and forward of the high speed lift center. FIG. 21 illustrates the behavior ofa pivot beam assembly where Y,, equals Y max. That is, 23 is halfway between the low speed lift center and the lift center position when h is maximum. Note that the control angle increases at first because the lift center is ahead of the pivot axis. The curve is flat topped because a mechanical stop 29 has been used limiting the control angle to a maximum positive value. As the lift center moves aft of the pivot axis, the control angle changes to negative valves of increasing magnitude.

It should be clear from the above discussion that stable behavior under rough sea conditions is dependent on proper positioning of pivot axis 23, and that the control mechanism responds not only to the changes in immersion depth but respond also to vertical accelerations and vertical shift of the center of drag on the foil. The control 'torqu-es arise from the large hydrodynamic forces on the main hydrofoils. The relatively small moment of inertia of the hydrofoilassembly about 23 permits fast response to these control torques.

I claim:

1. A hydrofoil sailing craft, comprising; a hull and a hydrofoil assembly coupled to said hull, said hydrofoil assembly including; an outrig means pivotally coupled to said hull; a pair of sweptback hydrofoils mounted on the two ends of said outrig means, respectively; and elastic restraint means attached to said outrig means and to said hull whereby said hydrofoil assembly pivots in a controlled manner about an axis parallel to and below the axis of said outrig means in response to the combined effects of said elastic restraint and the changing lift and drag forces acting on said hydrofoils as the speed and elevation of said craft change.

2. A craft in accordance with claim 1 wherein the elastic restraint is adjustable to provide a manual override of the control system.

3. A craft in accordance with claim 1 wherein the pivot axis of the hydrofoil assembly lies below the lowspeed water line of said craft.

4. A craft in accordance with claim 1 wherein the pivot axis of theoutrig means lies aft of the low speed lift center and forward of the high speed lift center on the main foil elements.

5. A craft in accordance with claim 1 wherein the pivot axis of the hydrofoil assembly is located a distance forward from the center of gravity of the hull equal to'between 6 and 24 percent of the distance from said center of gravityto the aft end of said hull.

6. A craft in accordance with claim 1 wherein the hydrofoils are swept back at an angle between about 8 and 25 degrees.

7. A craft in accordance with claim 1 wherein the hydrofoil assembly is coupled to the hull through pivot means, including, a pivot arm attached to the outrig means anda bearing attached to the hull of said craft.

8. A craft in accordance with claim 7 wherein the pivot arm carries a convex journal segment and a concavebearin g surface is attachedto the hull of said craft.

9. A craft in accordance with claim 1 wherein the hydrofiol assembly is coupled to the hull through a pivot beam assembly.

10. A craft in accordance with claim 9 wherein the pivot beam assembly includes two thin flexible beams projecting from each side of the hull of said craft and bracket means joining the-ou trig means to said beams.

11. A craft in accordance with claim 10 wherein the pivot beams are disposed at an angle with respect to one another so that their midplanes intersect on a line below said pivot beams and this line of intersection coincides approximately with the pivot axis of the hydrol as emb y,-

12. A craft in accordance with claim 10 wherein the angle between the flat sides of thev beams is between about 20 and degrees.

13. A craft in accordance with claim 9 wherein the pivot beam assembly performs a major portion of the fanst qn. f haslas im- 14. A craft in. accordance with claim 10 wherein the beams have a thickness less than about one-fifth of their width.

15. A craft in accordance with claim 1 wherein the hydrofoils are disposed at a dihedral with respect to the outrig means whereby the free ends of said hydrofoils project downwardly and inwardly toward the hull of said craft.

16. A craft in accordance with claim 13 wherein the dihedral angle of the hydrofoils is between about 30 and 50 d e g rees. MM g l 17. A craft in accordance with claim 1 wherein the hydrofoils are pivotally mounted on the outrig means and have a pivot axis substantially parallel to the long axis of the hull of said craft.

18. A craft in accordance with claim 1 whereinthe angle between the hydrofoils and the ourtrig means r ay be varied during operation of the craft.

19. A craft in accordance with claim 1 which also carries a rudder assembly.

top of the rudder assembly is slidably mounted to slide longitudinally with respect to the hull of said craft and said rudder assembly is held in operable position by the elastic cable system.

24. A craft in accordance with claim 19 wherein the rudder assembly is mounted through a double pivot assembly means to pivot on a vertical axis and an axis normal to the rudder of said rudder assembly.

25. A craft in accordance with claim 19 wherein the rudder assembly is adjustable to change the angle of attack of the rudder of said rudder assembly. 

1. A hydrofoil sailing craft, comprising; a hull and a hydrofoil assembly coupled to said hull, said hydrofoil assembly including; an outrig means pivotally coupled to said hull; a pair of sweptback hydrofoils mounted on the two ends of said outrig means, respectively; and elastic restraint means attached to said outrig means and to said hull whereby said hydrofoil assembly pivots in a controlled manner about an axis parallel to and below the axis of said outrig means in response to the combined effects of said elastic restraint and the changing lift and drag forces acting on said hydrofoils as the speed and elevation of said craft change.
 2. A craft in accordance with claim 1 wherein the elastic restraint is adjustable to provide a manual override of the control system.
 3. A craft in accordance with claim 1 wherein the pivot axis of the hydrofoil assembly lies below the low-speed water line of said craft.
 4. A craft in accordance with claim 1 wherein the pivot axis of the outrig means lies aft of the low speed lift center and forward of the high speed lift center on the main foil elements.
 5. A craft in accordance with claim 1 wherein the pivot axis of the hydrofoil assembly is located a distance forward from the center of gravity of the hull equal to between 6 and 24 percent of the distance from said center of gravity to the aft end of said hull.
 6. A craft in accordance with claim 1 wherein the hydrofoils are swept back at an angle between about 8 and 25 degrees.
 7. A craft in accordance with claim 1 wherein the hydrofoil assembly is coupled to the hull through pivot means, including, a pivot arm attached to the outrig means and a bearing attached to the hull of said craft.
 8. A craft in accordance with claim 7 wherein the pivot arm carries a convex journal segment and a concave bearing surface is attached to the hull of said craft.
 9. A craft in accordance with claim 1 wherein the hydrofiol assembly is coupled to the hull through a pivot beam assembly.
 10. A craft in accordance with claim 9 wherein the pivot beam assembly includes two thin flexible beams projecting from each side of the hull of said craft and bracket means joining the outrig means to said beams.
 11. A craft in accordance with claim 10 wherein the pivot beams are disposed at an angle with respect to one another so that their midplanes intersect on a line below said pivot beams and this line of intersection coincides approximately with the pivot axis of the hydrofoil assembly.
 12. A craft in accordance with claim 10 wherein the angle between the flat sides of the beams is between about 20 and 60 degrees.
 13. A craft in accordance with claim 9 wherein the pivot beam assembly performs a major portion of the function of the elastic restraint.
 14. A craft in accordance with claim 10 wherein the beams have a thickness less than about one-fifth of their width.
 15. A craft in accordance with claim 1 wherein the hydrofoils are disposed at a dihedral with respect to the outrig means whereby the free ends of said hydrofoils project downwardly and inwardly toward the hull of said craft.
 16. A craft in accordance with claim 13 wherein the dihedral angle of the hydrofoils is between about 30 and 50 degrees.
 17. A craft in accordance with claim 1 wherein the hydrofoils are pivotally mounted on the outrig means and have a pivot axis substantially parallel to the long axis of the hull of said craft.
 18. A craft in accordance with claim 1 wherein the angle between the hydrofoils and the ourtrig means may be varied during operation of the craft.
 19. A craft in accordance with claim 1 which also carries a rudder assembly.
 20. A craft in accordance with claIm 19 wherein the rudder assembly includes a rudder which carries a rigidly mounted lifting hydrofoil generally normal to said rudder.
 21. A craft in accordance with claim 20 wherein the lifting hydrofoil is mounted on the rudder a short distance below the planing surface of the stern of said craft.
 22. A craft in accordance with claim 20 wherein the rudder assembly is controlled through an elastic cable system.
 23. A craft in accordance with claim 22 wherein the top of the rudder assembly is slidably mounted to slide longitudinally with respect to the hull of said craft and said rudder assembly is held in operable position by the elastic cable system.
 24. A craft in accordance with claim 19 wherein the rudder assembly is mounted through a double pivot assembly means to pivot on a vertical axis and an axis normal to the rudder of said rudder assembly.
 25. A craft in accordance with claim 19 wherein the rudder assembly is adjustable to change the angle of attack of the rudder of said rudder assembly. 